1. Field of the Invention
The present invention relates to a method and apparatus for producing a semiconductor film, photoelectric conversion devices and method for producing the devices. More particularly, the present invention is related to a method and apparatus for producing a semiconductor film having a chalcopyrite structure, photoelectric conversion devices and method for producing the devices.
2. Description of Related Art
A thin-film solar cell provided with a photovoltaic absorber layer including a semiconductor film having a chalcopyrite structure including a group Ib element, a group IIIb element and a group VIb element, which is represented by the formula: CuInSe2, or such a semiconductor film having Ga and/or S intercrystallized, which is represented by the formula: Cu(In1-xGax)(SeyS1-y)2 (0≦x≦1, 0<y≦1), exhibits a high energy conversion efficiency. Therefore, such a thin-film solar cell is advantageous in that improvement of the conversion efficiency called light-soaking effects is achieved, and the thin-film solar cell exhibits an excellent resistance to aged deterioration. Generally, these types of thin films are referred to as CIS thin films or CIGS thin films, wherein the name represents the capital letters of the contained elements. Further, solar cells using such thin films are referred to as CIS solar cells or CIGS solar cells.
FIG. 1 shows a typical structure of a CIGS solar cell. This solar cell has a basic structure in which a back electrode layer 2, a CIGS photovoltaic absorber layer 3, a buffer layer 4, a highly resistive zinc oxide layer 5, a transparent electrode layer 6 and an antireflection layer 7 are sequentially laminated on a substrate 1. Each of the back electrode layer 2 and the transparent electrode layer 6 has an electrode formed thereon for taking out the current obtained by photoelectric conversion. As the substrate 1, glass may be used, or alternatively, a metal foil or a polymer may be used. The back electrode 2 is typically Mo. As the buffer layer 4, an n-type semiconductor film is used. As the transparent electrode layer 6, an indium oxide film or a zinc oxide film is used.
As representative methods for producing CIGS films, the selenization method and the multi-source evaporation method are known.
The selenization method is a method in which a metal precursor such as Cu or indium is heat-treated in a selenium gas prior to lamination, followed by forming a CIGS thin film. Specific examples of the selenization method include methods disclosed in U.S. Pat. Nos. 4,798,660, 4,915,745 and 5,045,409. Although the selenization method is known as a technique for manufacturing CIGS solar cells with a large area, it has problems in that a high conversion efficiency cannot be obtained.
On the other hand, as shown in FIG. 2, in the multi-source evaporation method, each of the raw materials are heated to a predetermined temperature to control the amount of evaporation, and a film is formed on a substrate (for example, see Thin Solid Films 403-404 (2002) p. 197-203, and U.S. Pat. Nos. 5,436,204 and 5,441,897). The multi-source evaporation method is advantageous in that a high conversion efficiency can be obtained by a laboratory-scale CIGS solar cell having a small area. However, in the multi-source evaporation method, with respect to the selenium vapor generated by heating of the source selenium (Se), the vapor pressure thereof has to be made several ten times larger than that of the source metals such as Cu, In and Ga. The reasons for this is as follows. In the multi-source evaporation method, an ordinary selenium vapor includes relatively large molecules such as Se2, Se5, Se6, Se7 and Se8, and hence, the reactivity thereof is low. Further, in the multi-source evaporation method, it is necessary to produce a film at a high temperature, and hence, selenium is re-vaporized from the surface of the produced film. Therefore, the amount of selenium contributing to the actual production of a film is extremely small as compared to the amount of selenium supplied to the vapor source crucible, and most of the supplied selenium is adhered to the inner wall or the like of the deposition chamber, and not utilized. As a result, a problem arises in that the utilization efficiency of the source selenium is markedly low as compared to other source metals. Thus, in the multi-source evaporation method, it was necessary to supply source selenium in an amount several ten times larger than that of source metals such as Cu, In and Ga, and to frequently perform maintenance such as removing substances adhered to the inner wall of the deposition chamber. Hence, the multi-source evaporation method was unsuitable for mass production.